@idefix.

Try this

$dateField = new DateField('Date');
$dateField->setConfig('showcalendar', true);
$dateField->setValue('PRESET_VALUE_IN_HERE');
$fields->addFieldToTab("Root.Content.Main", $dateField);

@Blackdog

Thanks for the hint.

The preset value appears, but not the calendar!
But I want both.

Have you tried the following?

new DateField('Date','Date','PRESET IN HERE');

Sorry, Calendar is now here, but no more the preset value!

Hi Blackdog,

This:

$ende = new DateField('Ende','Ende', '31/10/2010');
$ende->setConfig('showcalendar', true);
$fields->addFieldToTab("Root.News", $ende);

is the same as before, Calendar yes, Preset value is not visible and not even preselected in the calendar.

Is there any special way to format the date?

See http://www.silverstripe.org/dataobjectmanager-module-forum/show/15755#post298748

Hi,
I solved my problem:

to prefill a datefield you have to set the DataObject-variable.
e.g.:

function getCMSFields() {
$fields = parent::getCMSFields();
$today = date("d/m/Y");
$fldCalendar = new DatePickerField("Start","Start-Datum",$value="$today"); // $value="$today" doesn't take effect but in TextField it DOES!
if (!$this->Start) $this->Start = $heute; // THIS is the important thing! to show in the new created DataObject Form a prefilling value
$fields->addFieldToTab("Root.Content", $fldCalendar);
return $fields;
}

Maybe somebody can save some time with this.